Tchanghee Hong

Bio: Tchanghee Hong is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Semiconductor laser theory & Light intensity. The author has an hindex of 3, co-authored 3 publications receiving 201 citations.

Reduction of resonancelike peak in direct modulation due to carrier diffusion in injection laser.

Abstract: It is shown theoretically that the relaxation oscillation in the direct modulation of the injection laser is reduced by transverse carrier diffusion when the stripe width is comparable to the carrier diffusion length. Modified rate equations are derived by considering the distribution of the light intensity, the carrier density, and the diffusion of the carrier. By small-signal analysis of the rate equations it is found that the height of the resonancelike peak in the modulation characteristics is a minimum when the transverse width in which both the light and the carriers are confined is around the diffusion length.

Measurement of spontaneous-emission factor of AlGaAs double-heterostructure semiconductor lasers

Abstract: In this paper, experimental determination of the spontaneous-emission factor of an injection laser is presented. For a lot of conventional AlGaAs DH stripe lasers, spontaneous-emission factors of about 10-5were obtained from the measurement of the light intensity of a lasing mode versus the injected current. These values are in good agreement with the theoretical prediction based upon the classical wave theory. Brief discussion of the magnitude of the spontaneous-emission factor is given relating to the direct modulation characteristics of a semiconductor laser, which shows that the damping oscillation can be decreased in a laser consisting of very small active region.

Suppression of relaxation oscillation in light output of injection lasers by electrical resonance circuit

Abstract: It is shown that relaxation oscillation in the light outputs of injection lasers can be suppressed by an external electrical circuit. This method can be applicable to high-speed pulse modulation. For this purpose, an electrical resonant circuit connected to the laser diode (LD) and tuned to the relaxation oscillation is considered as an example. Frequency response of direct modulation and the device-transient characteristics are analyzed. These results were verified experimentally. A brief discussion on the upper limit of the repetition frequency of pulse modulation is given and it is shown theoretically that stable pulse modulation of an LD up to a few Gbit/s may be obtained by applying this method.

High-speed modulation of semiconductor lasers

Abstract: An overview is given of the direct modulation performance of high-speed semiconductor lasers. The high-speed response characteristics are described using a cascaded two-port model of the laser. This model separates the electrical parasitics from the intrinsic laser and enables these subsections to be considered separately. The presentation concentrates on the small-signal intensity modulation and frequency modulation responses, and the large-signal switching transients and chirping. Device-dependent limitations on high-speed performance are explored and circuit modeling techniques are briefly reviewed.

Analysis of gain suppression in undoped injection lasers

Abstract: This paper gives an analysis and discussion of gain suppression in injection lasers which have an undoped active region and an index guiding structure. In previous papers, we used a semiclassical density‐matrix analysis to show that an injection laser with an updoped active region has a nearly, but not perfectly, homogeneous (or uniform) gain property under operating conditions due to the mode coupling effects by phase synchronization of electrons to the lasing field. The gain of adjacent modes is well suppressed by the oscillating mode, and single‐longitudinal‐mode operation is obtained in undoped injection lasers. However, such suppression depends closely on the spacial distribution of the resonating field and injected carrier density. In this paper, the suppression effect is examined theoretically considering electronic intraband relaxation, effects from the standing wave of the lasing field, spatial diffusion of carriers, etc. When the relaxation time is larger than 3×10−13 sec, the gain shows ’’hole burning,’’ namely, strong nonuniformity across the spectral or energy distributions, and, at the same time, the gain of some resonating modes is increased. Single‐longitudinal‐mode operation is not obtained in such a strongly inhomogeneous laser. When the relaxation time is smaller than 2×10−13 sec, the gain can be seen to be nearly homogeneous, and the gain of nonoscillating modes is sufficiently suppressed, that is, lower than that at threshold, because of the strong‐mode‐coupling effect. The relaxation time of GaAs is expected to be approximately 1×10−13 sec, implying 0.1% of excess suppression. The spatial distribution of the resonating field and induced ’’spatial hole burning’’ of carriers tends to increase the gain of higher transverse modes, but only weakly affects the fundamental transverse modes when the oscillating mode is the fundamental mode. It is then necessary to design the laser so that such higher transverse modes are cut off. The results of this analysis explain the experimental data of well‐designed lasers which have an undoped active region and an index guiding structure. Such a gain suppression effect is crucial for the operation of injection lasers in a single‐longitudinal mode.

Density-matrix theory of semiconductor lasers with relaxation broadening model - Gain and gain-suppression in semiconductor lasers

Abstract: The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Density-matrix theory of semiconductor lasers with relaxation broadening model-gain and gain-suppression in semiconductor lasers

Abstract: The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Ultra-high speed semiconductor lasers

Abstract: Recent progress on semiconductor lasers having a very high direct modulation bandwidth of beyond 10 GHz will be described. Issues related to application of these lasers in actual systems will be addressed. Possibilities of further extending the bandwidth of semiconductor lasers will be examined.